Phase tuning in waveguide arrays

ABSTRACT

The wavelength response of an arrayed waveguide grating can be tuned, in accordance with various embodiments, using a beam sweeper including one or more heaters to shift a lateral position of light focused by the beam sweeper at an interface of the beam sweeper with an input free propagation region of the arrayed waveguide grating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/541,462, filed Aug. 15, 2019, which is a continuation of U.S. patentapplication Ser. No. 16/274,465, filed Feb. 13, 2019, which is acontinuation of U.S. patent application Ser. No. 15/988,015, filed May24, 2018, which claims priority to U.S. patent application Ser. No.15/838,667, filed Dec. 12, 2017, which claims priority to U.S. patentapplication Ser. No. 15/148,862, filed May 6, 2016, which claims benefitto U.S. Provisional Application No. 62/196,437, filed on Jul. 24, 2015,all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This disclosure relates to the tuning of the relative phases betweenwaveguides in an array, as applicable, for instance, in the field ofphotonic integrated circuits (PICs).

BACKGROUND

In wavelength division multiplexed systems, arrayed waveguide gratings(AWGs) are often used as optical multiplexers or demultiplexers. Withreference to FIG. 1, an AWG 100 generally includes an array 102 ofwaveguides 103 typically having constant optical path length incrementsbetween adjacent waveguides 103, connected between an input freepropagation region (FPR) 104 and an output FPR 106. When the AWG 100operates as a wavelength demultiplexer, as depicted, input lightincluding multiple wavelengths diffracting out of an input waveguide 108or other input coupler into the input FPR 104 propagates through theinput FPR 104 to illuminate the input ports of the array 102 ofwaveguides 103. After propagating through the array 102 of waveguides103 and accumulating different optical phases in different ones of thewaveguides 103 due to the different respective optical path lengths,light exiting the array 102 of waveguides 103 at their output ports isrefocused in the output FPR 106, whereby light of different wavelengthsconstructively interferes, and thus refocuses, at different locations. Aplurality of output waveguides 110 or other output couplers may beplaced at the various foci so as to capture light of the respectivewavelengths. To operate the AWG 100 as a multiplexer, the direction oflight propagation through the AWG 100, and thus the roles of the FPRs104, 106, can be reversed: light of multiple wavelengths is coupled frommultiple respective waveguides 110 into the FPR 106 (which therebyfunctions as the input to the AWG 100) and dispersed in the FPR 106 toilluminate the array 102 of waveguides 103, and after propagatingthrough the array 102 of waveguides 103, the now mixed-wavelength lightfrom all of the arrayed waveguides 103 is refocused in the FPR 104 (nowfunctioning as the output of the AWG), from where it exits into thewaveguide 108.

When implemented in PICs, AWGs are susceptible to a number of factorsthat can affect their wavelength response, often resulting in themismapping of wavelengths to output waveguides. For example, due tofabrication tolerances of PICs, the effective index of the waveguides inthe array may not be controlled accurately enough to achieve an intendedwavelength response. Fabrication-based deviations from the intendedresponse are especially likely if the waveguide dimensions are small, ifthe AWG waveguide core is in a deposited layer for which thicknesscontrol is poor, or if the refractive index of the waveguide core isdependent upon material growth or deposition conditions. In addition tothese problems, the effective index of the waveguides in the array, andthus the wavelength response of the AWG as a whole, varies as a functionof temperature. This effect is particularly pronounced where waveguidecore materials having a large thermooptic coefficient, such as silicon,are used, and tends to limit the temperature range over which the AWGcan be used. One approach to reducing undesirable wavelength shifts ofthe AWG response due to fluctuations in the ambient temperature, and/orto compensating for fabrication-based deviations from the desiredwavelength response, involves actively controlling the temperature ofthe AWG, which, however, requires a significant amount of power,rendering the PIC less efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top view of an AWG as may be used in variousembodiments.

FIG. 2A is a top view of a beam sweeper coupled to the input FPR of anAWG, in accordance with various embodiments.

FIG. 2B is a top view of a system including a bidirectional AWG and beamsweepers coupled to both FPRs of the AWG, in accordance with variousembodiments.

FIG. 3 is a close-up top view of the coupling region between the beamsweeper and the AWG of FIG. 2A, in accordance with various embodiments.

FIG. 4 is another top view of the beam sweeper of FIG. 2, furtherillustrating the placement of heating regions in straight waveguidesections, in accordance with various embodiments.

FIG. 5 is a top view of an AWG having heaters disposed above straightsections of the waveguides, in accordance with various embodiments.

FIG. 6 is a cross-sectional view of an SOI substrate having amulti-waveguide structure and associated heater and temperature-sensingelements implemented therein, in accordance with various embodiments.

FIG. 7 is a top detail view of heaters disposed above a straightwaveguide section, in accordance with various embodiments.

FIG. 8 is another top detail view of the heaters of FIG. 7, furtherillustrating the placement of temperature-sensing elements and thermalisolation trenches in accordance with various embodiments.

DETAILED DESCRIPTION

Described herein, in various embodiments, are systems, devices, andstructures for tuning the relative phases between waveguides in an arrayby the active, controlled application of heat to the waveguides, forexample, for the purpose of tuning the wavelength response of AWGs orother dispersive gratings. In some embodiments, one or more heaters aredisposed directly in an AWG in the vicinity of the waveguides (forinstance, above, in between, or even partially overlapping with thearrayed waveguides) to impart an incremental heat-induced phase shiftbetween the waveguides. (By “incremental phase shift between thewaveguides” is herein meant that the phase shifts between adjacentwaveguides are all in the same direction such that the cumulative phaseshift changes monotonically across the array. In many but notnecessarily all embodiments, the increments between adjacent waveguidesare constant across the array.) In other embodiments, a separatemulti-waveguide structure coupled to and focusing light onto the inputFPR of an AWG includes one or more heaters to control the relativephases between the waveguides of the separate structure so as to shift alateral position of the focus, thereby altering the wavelength responseof the AWG; such a separate multi-waveguide structure is herein called a“beam sweeper” (or simply “sweeper”). (The “lateral position” hereindenotes a position along a direction perpendicular to the generaldirection of propagation.)

Using a beam sweeper at the input of an AWG instead of a heater directlyplaced in the AWG is beneficial, in particular, in embodiments where theAWG includes a large number of waveguides, rendering the direct heatingof the AWG waveguides energy-costly. A separate beam sweeper that hasfewer waveguides than the AWG itself may allow for more efficient tuningof the AWG wavelength response. Beam sweepers in accordance herewith mayalso be used with dispersive gratings other than AWGs, such as, e.g.,Echelle gratings or vertical grating couplers (where the beam sweepermay be used to tune the direction of the coupling beam). Furthermore, abeam sweeper as disclosed herein may find application apart from anydispersive gratings, e.g., in optical switching. Heaters used in AWGs orbeam sweepers in any of these embodiment may be controlled based on thewaveguide temperature as measured with suitable temperature-sensingelements.

In the context of AWGs, the terms “input FPR” and “output FPR” areherein used, for definiteness and to tie them to structure rather thanfunction, with reference to an AWG operating as a demultiplexer. Withthese designations, operation of an AWG as a multiplexer involves lightentering the AWG at the output FPR and exiting the AWG at the input FPR.In this manner, the term “input FPR” is used consistently to refer tothe FPR into or out of which multiplexed (mixed-wavelength) light iscoupled, and the term “output FPR” is used consistently to refer to theFPR out of or into which demultiplexed light (light of multipleseparated wavelengths) is coupled; structurally, an output FPR may bedistinguished from the input FPR, e.g., by virtue of the multiple outputwaveguides (e.g., with reference to FIG. 1, waveguides 110) emanatingfrom it. Further, in embodiments including a sweeper and AWG, thesweeper is consistently coupled to the input FPR of the AWG, regardlesswhether the AWG is used as a demuliplexer (in which case lightpropagates through the sweeper before entering the AWG) or a multiplexer(in which case light propagates through the AWG before entering thesweeper). Note that, in accordance with some embodiments, the AWG can bebidirectional in that it exhibits a symmetry that allows it to functionas either multiplexer or demultiplexer in either direction. In thiscase, the FPRs on both sides of the array of waveguides function as bothinput and output FPRs, e.g., by being each connected to both an inputwaveguide and multiple output waveguides. To tune the wavelengthresponse of such a bidirectional AWG, beam sweepers may be coupledsymmetrically to both input/output FPRs.

In accordance with various embodiments, the AWG and/or beam sweeper andassociated heaters are implemented as part of a PIC in an SOI substrateincluding a silicon handle, a buried oxide layer disposed on top of thesilicon handle, a silicon device layer disposed on top of the buriedoxide layer, and a cladding layer disposed on top of the silicon devicelayer. For instance, waveguides and FPRs may be formed in the silicondevice layer, and heaters and temperature-sensing elements may beembedded in the cladding layer. Alternatively, segments of a heater maybe created directly in the silicon device layer, e.g., placed betweenthe waveguides, by doping the silicon to alter is electrical resistance.Various structural features may serve to increase the efficiency ofwavelength-response tuning via heat (as may be measured, for example, interms of the power used to achieve a certain phase shift or, in the caseof an AWG, wavelength shift). For example, a back-etched region in thesilicon handle in a region underneath a heater may eliminate asignificant portion of the heat sink that the silicon handle otherwiseconstitutes, substantially reducing heat dissipation into the siliconhandle. Since removal of the silicon handle can result in mechanicalstresses on the waveguides, which may, in turn, affect the wavelengthresponse, the width of the back-etched region may be limited to limitthe amount of stress, and may further be constant across the waveguidesto ensure that any remaining stresses affect all waveguides uniformly(so as to avoid undesirable relative phase shifts). Heating efficiencymay also be increased with thermal isolation trenches formed in thesilicon device layer and/or the cladding layer surrounding the heatedwaveguide regions, which can serve to retain the generated heat in thoseregions and reduce heat dissipation into the substrate at large.Similarly, thermal isolation trenches may be formed within the heatedregion between heaters (or heater segments) and temperature-sensingelements to ensure that the generated heat is applied primarily to theadjacent waveguides rather than the temperature-sensing elements.

A further approach to increasing the wavelength-tuning efficiency ofAWGs and/or sweepers as described herein involves the use ofbidirectional pairs of heaters. A bidirectional pair of heaters includesfirst and second heaters that impart phase shifts of opposite signsbetween the waveguides such that, when light exiting the waveguides isfocused down (e.g., by a sweeper onto the input FPR of an AWG, or in theoutput FPR of an AWG onto output waveguides), the first heater causes ashift in the lateral focus position in a direction opposite to that of ashift in the lateral focus position caused by the second heater. Thefirst and second heaters are herein also referred to as the “forwardheater” and “backward heater,” respectively. (As will be readilyappreciated, at any given time, only one of the heaters of thebidirectional pair is operated, i.e., the forward and backward heatersare not operated simultaneously.) Using a bidirectional pair of heaters,a given total phase-tuning range (corresponding to the phase differencebetween the two extreme phases) can be achieved with heaters each ofwhich individually covers only half that range; this, in turn, reducesthe peak power consumption associated with the tuning range to about onehalf, compared with the peak power consumption of a single heaterachieving the same range.

In some embodiments, multiple heaters causing phase shifts in the samedirection are used in parallel, accumulating the phase shifts induced bythe individual heaters (which are subject to limits placed on the powerthat can be put through a heater) to achieve a greater total phaseshift. Using multiple heaters in parallel reduces the drive voltagerequirement associated with a given total phase shift, rendering theheaters deployable in systems where large drive voltages are notavailable and/or compatible with the electronics requirements of manystandard photonics packages (such as, e.g., Quad Small Form-factorPluggables (QSFPs)).

In various embodiments, the heaters are configured to impart a constantincremental phase shift between adjacent waveguides. For example, theheaters may be configured to generate a substantially uniformly heatedregion (as defined below) so as to impart a uniform phase shift per unitlength of heated waveguide, the heated region being shaped andpositioned such that heated waveguide portions increase, betweenadjacent waveguides, by constant length increments. For example, theheated region may be triangular in shape and overlap straight sectionsof evenly spaced waveguides. Alternatively, the waveguides may be spacedunevenly in the heated region, the spacings being designed based, e.g.,on a given temperature distribution achievable with the heater to resultin constant phase shifts between adjacent pairs of waveguides. In someembodiments, a heater is implemented by a current-carrying heatingfilament, e.g., made of platinum or tungsten (or some other suitablemetal), that is wound across an area defining the heated region. Theheating filament may have a constant width and/or cross-section withinthat area to simplify modeling of the resulting temperature distributionand/or facilitate more accurate results. As will be readily appreciatedby one of ordinary skill in the art, approximating a region to be heatedwith a heating filament wound across an area defining that region willgenerally result in some level of variation in the temperaturedistribution across the heated region; a region is herein considered“substantially uniformly heated” if such variation in temperature iswithin an acceptable range. In some embodiments, the acceptable range isdefined by ±10% of the average temperature within the region.

The foregoing will be more readily understood from the followingdetailed description of the drawings.

FIG. 2A is a top view of a beam sweeper 200 in accordance with variousembodiments, coupled to the input FPR 104 of an AWG 100. As shown, thebeam sweeper 200 includes an input FPR 202, an output FPR 204 adjoining,and thus optically coupled to, the input FPR 104 of the AWG 100, and anarray 206 of waveguides 207 connecting the input and output FPRs 202,204 of the sweeper 200. An input light signal may be provided to theinput FPR 202 of the sweeper 200 via an input waveguide 210. (Thedesignations of the FPRs 202, 204 of the sweeper 200 as “input FPR 202”and “output FPR 204” are used, consistently with the designations “inputFPR 104” and output FPR 106″ of the AWG 100, with reference to the AWG100 operating as a demultiplexer. That is, when the AWG 100 operates asa demultiplexer, the beam sweeper 200 precedes the AWG 100, and lightpropagates in the sweeper 200 from the input FPR 202 to the output FPR204. When the AWG 100 operates as a multiplexer, the beam sweeper 200follows the AWG 100 in the direction of light propagation, with lightentering the beam sweeper 200 at the output FPR 204 and exiting thesweeper 200 at the input FPR 202.)

FIG. 2B is a top view of a system including a bidirectional AWG 250 andbeam sweepers 200 coupled to both FPRs of the AWG 250, in accordancewith various embodiments. Here, the FPRs on either end of the waveguidearray 102 are structurally and functionally both input and output FPRs104/106. To be able to serve as output FPR, each of the FPRs 104/106 iscoupled to output waveguides 110. To be able serve as an input FPR, eachof the FPRs 104/106 is further coupled to the output FPR 204 of a beamsweeper 200 that receives its mixed-wavelength input light through aninput waveguide 210 at input FPR 202. In the illustrated embodiment, thesystem including the AWG 250 and the two beam sweepers 200 coupledthereto (as well as the AWG 250 itself) is symmetric.

With renewed reference to FIG. 2A, the waveguides 207 and output FPR 204of the sweeper 200 may be configured to focus light propagating in thewaveguides 207 from the input FPR 202 to the output FPR 204 of thesweeper 200 at an interface of the sweeper 200 with the AWG 100. Forexample, as shown in the close-up view of the coupling region betweensweeper 200 and AWG 100 provided in FIG. 3, the sweeper waveguides 207may be oriented, in a region immediately preceding the output FPR 204,along rays emanating from a common center point 300 at the interface302. Similarly, the AWG waveguides 103 may be oriented, in a regionimmediately following the input FPR 104 of the AWG 100, along raysemanating from the same (or substantially the same) center point 300.(The term “substantially” herein accounts for slight offsets between thecenter points corresponding to the rays along which the AWG waveguides103 are oriented and the rays along which the sweeper waveguides 207 areoriented, respectively, as may arise in practice, e.g., due tofabrication inaccuracies, and generally does not affect the functioningof the device.) Further, the entrance surface of the output FPR 204 ofthe sweeper 200 and the exit surface of the input FPR 104 of the AWG 100may each coincide with the circumference of a circle centered at point300, with the waveguides 207, 103 being perpendicular to the respectivesurface (due to their arrangement on rays emanating from the centerpoint 300).

In the absence of heat applied to the sweeper waveguides 207, lightexiting the waveguides 207 will be focused at the point 300, anddispersed from point 300 into the input FPR 104 of the AWG to illuminatethe input ports of the AWG waveguides 103. When the heater(s) of thesweeper 200 are used to impose a linear phase variation across the array206 of waveguides 207, the focus at the interface 302 is translatedlaterally (side-ways) away from the center point 300 (as indicated byarrows 304), resulting in an altered wavelength response of the AWG 100.As the total phase shift applied across the array 206 is tunedcontinuously between one extreme and the other (e.g., between zero andthe maximum phase shift for a single heater, or between the maximumphase shifts in either direction for a bidirectional pair of heaters),the focus traces a line along the interface 302.

The output FPR 204 of the sweeper 200 and the input FPR 102 of the AWG100 may be, and generally are when implemented in SOI substrates,contiguous. In the depicted embodiment, the FPRs 204, 104 are shaped toform a constriction or “waist” 306 at the interface 302. This waist 306can serve as a spatial filter that prevents light of higher diffractionorders from entering the input FPR 104 of the AWG 100; when the AWG 100is used as a demultiplexer, this may result in wavelength channelsseparated onto their respective output waveguides 110 with reducedcross-talk. The waist 306 also provides a demarcation between the FPRs204, 104. Note, however, that a waist between the FPRs 206, 104 need notbe formed in every embodiment. Rather, the contiguous region formed bythe FPRs 204, 104 may be devoid of a clear visual boundary between theFPRs 204, 104 of the sweeper 200 and AWG 100. Therefore, the interface302 is herein defined functionally as the focal plane at the output ofthe sweeper 200, that is, the vertical plane going through the focus atpoint 300 and oriented perpendicular to the general direction ofpropagation.

Returning again to FIG. 2A, in various embodiments, the waveguides 207of the beam sweeper 200 are equal in length, which avoidswavelength-dependent dispersion to ensure that light of all wavelengthsis focused in the same spot (about point 300) at the output of thesweeper 200. (Of course, once a phase gradient is applied across thearray 206 of waveguides 207, slight wavelength dispersion results.However, this effect is so small as to be negligible. Typically, themaximum heat-induced phase shift between two adjacent waveguides in thesweeper 200 amounts to merely a small fraction of the center wavelength.For comparison, the optical path difference between two adjacentwaveguides in an AWG may be on the order of tens of wavelengths.) Thedepicted configuration of equal-length waveguides is characterized by anarray consisting of two equal portions, corresponding to the portionsabove and below the horizontal line 260 through a symmetry center 262,that map onto one another if one portion is rotated by 180° about thesymmetry center 262.

When used in conjunction with an AWG 100, the sweeper 200 generallyincludes fewer waveguides than the AWG 100 in order to reduce the powerrequirements associated with tuning the wavelength response of the AWG100 (compared with the use of heaters included directly above thewaveguides 103 of the AWG 100). On the other hand, the waveguide array206 of the sweeper 200 is generally provided with a sufficient number ofwaveguides 207 to limit the insertion loss at the input FPR 202 to anacceptable level and achieve a desired image quality of the focusgenerated at the output of the sweeper 200 (that is, at the interface300 between the output FPR 204 and the input FPR 104 of the AWG 100).With too few waveguides 207 in the sweeper 200, a significant portion ofthe light from the input waveguide 210 may not be captured by thewaveguide array 206, and/or a significant portion of light at the outputmay be concentrated in side lobes of the focus rather than the centralfocus area. Usually, it is desirable to generate a focus that is to goodapproximation Gaussian. In various embodiments, the array 206 in thesweeper 200 includes at least three, and typically more (e.g., about15-30), waveguides 207.

Referring now to FIG. 4, the placement of heaters in the beam sweeper200, in accordance with some embodiments, is illustrated in a furthertop view. In general, the beam sweeper 200 may include one or moreheaters, for example, multiple heaters driven in parallel to obtain alarger overall phase-tuning range at a given drive voltage per heater,or one or more bidirectional pairs of heaters causing phase shifts inmutually opposite directions (indicated by “F” for forward heaters and“B” for backward heaters). In the illustrated embodiment, the sweeper200 includes two bidirectional pairs 400, 402 of heaters 404, 405, 406,407. The heaters are generally placed laterally overlapping (i.e.,overlapping in a top view) with the waveguides 207, in the same or adifferent layer. For instance, in some embodiments, the heaters areplaced above the waveguides 207 (see also FIG. 6), and in otherembodiments, the heaters are placed within the layer defining thewaveguides 207 (and potentially include portions of the waveguides 207).As shown, the heaters 404, 405, 406, 407 may be placed above (or within)straight sections of the waveguides 207, which may simplify thermalmodeling of the heat-induced phase shifts.

In accordance with various embodiments, the heaters are configured tocause a constant incremental phase shift between pairs of adjacentwaveguides 207. In principle, this can be achieved, for example, byuniformly heating a triangular region overlapping evenly spacedwaveguides, with one edge of the triangle being oriented in parallelwith the waveguides such that the length of the waveguide portionsoverlapping with the triangular region increases by a constant incrementbetween adjacent waveguides. In conjunction with a uniform phase shiftper unit length of heated waveguide, as results from a uniformtemperature across the heated region, this configuration can achieve thedesired constant incremental phase shift. In practice, however, it maybe difficult to heat the triangular region sufficiently uniformly. Inthis case, the spacings between the waveguides may be adjusted, based onthermal modeling, to achieve constant phase increments despite thenonuniform temperature distribution. The geometry of the waveguide array206 shown in FIGS. 2 and 4 facilitates modifying the waveguide spacingsas desired without affecting the overall length of any waveguide,thereby maintaining equal lengths of all of the waveguides 207. Forexample, to increase the spacing between the top waveguide 410 and itsadjacent waveguide 412 in the straight waveguide region 415, the lengthof the straight section of the top waveguide 410 in the top region 420may be decreased, and the length of the straight section of the topwaveguide 410 in the bottom region 425 increased, by the same distance.The spacing between any other pair of waveguides 207 can be adjustedsimilarly by modifying the length of one of the waveguides by equal butopposite-signed amounts in the top and bottom regions 420, 425.

FIG. 5 is a top view of an example AWG 500 having heaters disposed abovestraight sections of the waveguides 103, in accordance with variousembodiments. Here, the wavelength response of the AWG 500 is tuneddirectly by heat-induced phase shifts imparted on the waveguides 103 ofthe AWG 500, rather than by a separate beam sweeper 200 at the input FPR104 of the AWG 500. Various features of heaters included in a beamsweeper 200, as described above, are equally applicable for the heatersin an AWG. For instance, the AWG may generally include one or moreheaters, and may use one or more bidirectional pairs of forward andbackward heaters and/or multiple heaters driven in parallel. In theillustrated example, the AWG 500 includes four bidirectional pairs ofheaters 502, 503, 504, 505. As shown, the bidirectional pairs of heaters502, 503, 504, 505 are arranged symmetrically above straight sections ofthe array 102 waveguides 103. Beneficially, heater placement abovestraight sections allows using identical designs for the forward andbackward heaters while achieving maximum phase shifts of equal magnitudein each direction. Note, however, that heaters can generally also beplaced above the curved sections of the array 102 of waveguides 103.

FIG. 5 further illustrates the configuration and design of theindividual heaters in more detail. As shown, each heater (e.g., forwardheater 510 of pair 502, indicated by an enclosing dashed line) mayinclude multiple filamentous resistive heating segments 512geometrically arranged in parallel and having suitable lengths tocollectively define a heated region of a desired shape. In theillustrated embodiment, for example, the heating segments 512 of eachheater collectively define a triangular region. Within each individualheater, the heating segments 512 are connected in series by metalconnections 514 so as to effectively form a single heating filamentwound across the heated region. A voltage applied between the electricalconnection nodes (labeled F+ and F− for the two polarities of theforward heater and B+ and B− for the two polarities of the backwardheaters) at opposite ends of the heating filament causes an electricalcurrent that resistively heats the filament. Note, however, that theheating segments 512 and the metal connections 514 therebetween maydiffer in their cross-sectional dimensions and material properties and,as a result, their respective electric resistances. For example,low-resistance metal connections 514 may be used with higher-resistanceheating segment 512 such that, as an electric current flows through thefilament, heat is generated preferentially in the heating segments 512.The forward heaters may be driven in parallel by connecting theirrespective positive connection nodes F+ to one another and connectingtheir respective negative connection nodes F− top one another.Similarly, the backward heaters may be driving in parallel by connectingrespective nodes of the same polarities to one another.

In accordance with various embodiments, thermal modeling is used tocompute, based on the configuration of the heating filament in a heater,the resulting temperature distribution created by the heater. From thisdistribution, the phase shifts imparted on the waveguides can, in turn,be computed. In some embodiments, the heating filaments, or at least theheating segments therein, have a constant width and cross-section, whichsimplifies the modeling. The heater and waveguides collectively may beconfigured to achieve a desirable constant incremental phase shiftbetween pairs of adjacent waveguides. For example, a triangular heatedregion of uniform temperature (if achievable) may be used in conjunctionwith evenly spaced waveguides. For non-uniform temperaturedistributions, the spacing of the waveguides underneath the heater maybe adjusted; for example, if the temperature falls off from a maximumtemperature in accordance with an approximately exponential profile, thespacing between waveguides may be increased towards the exponentialtail.

Turning now to the implementation of AWGs and beam sweepers as describedabove in PICs, FIG. 6 is a cross-section of an example SOI substrate 600having a multi-waveguide structure and associated heating segments andtemperature-sensing elements implemented therein, in accordance withvarious embodiments. The SOI substrate 600 includes a silicon handle602, a buried oxide (BOX) layer 604 disposed above the handle 602, asilicon device layer 606 on top of the BOX layer 604, and a dielectriccladding layer 608 above the silicon device layer 606. A plurality ofsilicon rib waveguides 610 are formed in the silicon device layer 606.The space between the rib waveguides 610 (e.g., created by etching) maybe filled with dielectric material (e.g., the same material as used forthe cladding layer 608). The rib waveguides 610 may implement thearrayed waveguides 103, 207 of an AWG or beam sweeper as describedherein.

In the example embodiment of FIG. 6, the heaters are implemented byheating segments 512 disposed above the waveguides 610 in the claddinglayer 608. Alternatively or additionally to being disposed in the layer608 above the waveguides 610, heating segments 512 may also be createdwithin the silicon device layer 606 (not shown), e.g., by doping thesilicon to render it resistive; in this case, the heating segments 512may be placed between the waveguides 610 or even include portions of (inother words, at least in part spatially overlap with) the waveguides610. In either case, the heating segments 512 overlap laterally with thewaveguides, meaning that their projections into a horizontal planeoverlap. Further, as shown, temperature-sensing elements 614, e.g.,forming the segments of a resistive thermal device (RTD), may beembedded in the cladding layer 608 to directly measure the temperaturein the heated region. The temperature-sensing elements 614 and heatingsegments 512 may be arranged in an alternating fashion.

Based on the temperature within a heated region as measured bytemperature-sensing elements 614, the temperature-dependence of thephase shift imparted by deliberately heating the waveguides (or thetemperature-dependence of a resulting optical characteristic such as, ina sweeper, the lateral shift in the focus at the sweeper output) can becalibrated. Alternatively, the phase shift as a function of the voltageor current applied to the heater may be calibrated (obviating, in someembodiments, the need for sensing the temperature of the heated region).To compensate for an undesired phase shift of the AWG 100 due to achange in the ambient temperature, the temperature of the waveguides 103of the AWG 100 in a region outside the heater may be measured todetermine therefrom how much tuning is needed. Accordingly, in variousembodiments, temperature-sensing elements are included both within andnear the heated region (in the sweeper or a directly heated waveguidesection of the AWG, as the case may be) and in a region away from theheater.

With renewed reference to FIG. 6 and reference further to FIG. 7, whichprovides a top detail view of two heaters disposed above a straightsection of waveguides 700, various features directed at increasing theefficiency of thermal phase-tuning are illustrated. One such feature isa back-etched region 620 in the silicon handle 602, located directlyunderneath the heated waveguide region. Silicon removal underneath theheater serves to eliminate heat dissipation into the silicon handle 602within the back-etched region 620, which helps to retain more heat inthe heated region of the silicon device layer 606. In accordance withvarious embodiments, silicon is fully removed within the boundaries ofthe back-etched region 620, without leaving any silicon “islands,” so asto maximize the effect of the back-etch. On the other hand, in order toavoid an unnecessary loss in structural stability of the SOI substrate600, silicon removal from the handle 602 is confined, in accordance withvarious embodiments, to a region 620 substantially coinciding with anarea defined by the heater (e.g., not exceeding maximum lateraldimensions of the heater by more than 20%). This is illustrated moreclearly in FIG. 7. As shown, the back-etched region 620 may, forexample, be within the outer limits of the heating segments 512 in thedirection of the waveguides 700 and extend only slightly beyond theouter limits of the heating segments 512 in the direction perpendicularto the waveguides 700 (e.g., such that the dimension of the back-etchedregion 620 measured perpendicularly to the waveguides 700 exceeds thatof the heater by no more than 20%).

Besides the effect on the overall structural stability of the SOIsubstrate 600, another concern associated with a back-etched region 620underneath the waveguides 610, 700 are mechanical stresses imposed onthe waveguides due to the nonuniformity of the substrate. Limiting thewidth 710 of the back-etched region 620, measured in the direction ofthe waveguides 700, can minimize this problem. In some embodiments, thewidth 710 of the back-etched region 620 is less than 40 μm; forcomparison, typical dimensions of the waveguide array within an AWG maybe on the order of 200 μm×200 μm. Thus, the back-etched region 620 issignificantly smaller in area than the waveguide array. In addition, theeffect of any remaining mechanical stresses on the phase of thewaveguides can be neutralized by ensuring that all waveguides experiencethe same stress level. This, in turn, may be achieved by using aback-etched region 620 that has a constant width 710 across the array ofwaveguides (e.g., that is rectangular in shape) and placing the heater,and thus the back-etched region, in a region of the waveguide arraywhere the waveguides are straight and/or have constant curvature(straight waveguides corresponding to zero curvature), or by otherwiseensuring that the length of waveguide overlapping the back-etched region620 is the same for all waveguides.

In addition to removing the heat sink underneath the heater, the tuningefficiency may be increased by thermally isolating the heated regionfrom surrounding regions in the silicon device layer. For this purpose,various embodiments include thermal isolation channels (or, if open atthe top, “trenches”) 622 in the silicon device layer 606 (as shown)and/or the dielectric cladding layer 608 (not shown) alongside thewaveguides 610, 700 on either or both sides of the waveguide array,surrounding the heated portions of the waveguides. Further, inembodiments that include temperature-sensing elements 614 adjacent(e.g., arranged alternatingly with) the heating segments 512, thermalisolation trenches 624 may be included between adjacent heating andtemperature-sensing elements 512, 614 such that the heat is appliedprimarily to the waveguides 610 rather than the temperature-sensingelements 614.

FIG. 8 is another detail view of the heaters of FIG. 7, showing theplacement of temperature-sensing elements 614 and thermal isolationtrenches in accordance with various embodiments. The view corresponds toa horizontal cross-section taken through the dielectric cladding layer608, overlaid onto a top view of the waveguides 700. As shown, thetemperature-sensing elements 614 are placed between the heating segments512 in parallel therewith, and are serially connected, by metalconnections, to form a continuous temperature-sensing filament 800winding across the area defined by both heaters. Thermal isolationtrenches 624 line the sides of portions of the temperature-sensingfilament 800, in particular, in regions between the heating segments 512and the temperature-sensing elements 614.

The following numbered examples are illustrative embodiments.

1. A PIC comprising: a silicon-on-insulator (SOI) substrate comprising asilicon handle, a buried oxide layer disposed on top of the siliconhandle, a silicon device layer disposed on top of the buried oxidelayer, and a cladding layer disposed on top of the silicon device layer;formed at least partially within the silicon device layer, an AWGcomprising an array of waveguides connected between an input FPR and anoutput FPR, the waveguides varying in optical path length varying inoptical path length incrementally across the array to thereby causewavelength dispersion; disposed at least partially within at least oneof the cladding layer or the silicon device layer, one or morebidirectional pair of heaters laterally overlapping with the array ofwaveguides and configured to impart an incremental heat-induced phaseshift between the waveguides to thereby alter the wavelength dispersionof the AWG; and one or more back-etched regions formed within thesilicon substrate underneath respective one or more bidirectional pairof heaters.

2. The PIC of example 1, wherein the back-etched region has a constantwidth across the array of waveguides.

3. The PIC of example 2, wherein the constant width does not exceedforty micrometers.

4. The PIC of any of the preceding examples, wherein the one or moreback-etched region are confined to regions substantially coinciding withareas defined by the respective one or more bidirectional pairs ofheaters.

5. The PIC of any of the preceding examples, wherein the heaterslaterally overlap with a straight section of the array of waveguides.

6. The PIC of any of examples 1-4, wherein the heaters laterally overlapwith a section of the array of waveguides in which the waveguides allhave a common curvature.

7. The PIC of any of the preceding examples, further comprisingthermal-isolation channels formed in at least one of the silicon devicelayer or the cladding layer surrounding heated regions of the array ofwaveguides.

8. The PIC of any of the preceding examples, wherein each of the heaterscomprises a heating filament winding across an area laterallyoverlapping with a region of the array of waveguides to be heated.

9. The PIC of example 8, wherein the heating filaments comprise heatingsegments of equal and constant width connected by metal connections.

10. The PIC of example 8, further comprising temperature-sensingelements embedded in the cladding layer between heating segments of theheating filament.

11. The PIC of claim 10, further comprising thermal isolation trenchesformed in the cladding layer between adjacent heating segments andtemperature-sensing elements.

12. The PIC of any of examples 1-9, wherein each of the heaters comprisedoped, resistive regions in the silicon device layer.

13. The PIC of any of the preceding examples, comprising a plurality ofbidirectional pairs of heaters configured to operate in parallel.

14. The PIC of any of the preceding examples, wherein each of theheaters is configured to impart a constant incremental phase shiftbetween all pairs of adjacent waveguides of the array.

15. The PIC of example 14, wherein each of the heaters is configured toheat a heated region spatially overlapping with the array of waveguidesto a substantially uniform temperature so as to impart a uniform phaseshift per unit length of heated waveguide, the heated region beingshaped and positioned such that heated waveguide portions increase,between all pairs of adjacent waveguides, by constant length increments.

16. The PIC of example 15, wherein the waveguides are uniformly spacedin the heated region and the heated regions are triangular in shape.

17. The PIC of any of examples 14-16, wherein spacings between thewaveguides are selected based on a temperature distribution associatedwith the heaters to achieve the constant incremental phase shift.

18. A system comprising: an AWG comprising a plurality of waveguidesconnected between a first input FPR and a first output FPR; and a beamsweeper for tuning a wavelength response of the AWG, the beam sweepercomprising a second input FPR, a second output FPR adjoining andoptically coupled to the first input FPR of the AWG, at least threewaveguides connected between the second input FPR and the second outputFPR, the at least three waveguides and the second output FPR beingconfigured to focus light propagating in the at least three waveguidesfrom the second input FPR to the second output FPR at a focus at aninterface between the second output FPR and the first input FPR, and atleast one bidirectional pair of heaters laterally overlapping with theat least three waveguides, each of the heaters configured to impart anincremental phase shift between the light propagating in the at leastthree waveguides to thereby shift a lateral position of the focus.

19. The system of example 18, wherein the at least three waveguides ofthe beam sweeper are fewer in number than the waveguides of the AWG.

20. The system of example 18 or example 19, wherein the at least threewaveguides of the beam sweeper are arranged, in a region immediatelypreceding the second output FPR, along rays emanating from a commoncenter at the interface of the second output FPR with the first inputFPR.

21. The system of example 20, wherein the plurality of waveguides of theAWG are arranged, in a region immediately following the first input FPR,along rays emanating from the center.

22. The system of any of examples 18-21, wherein the at least threewaveguides of the beam sweeper are all equal in length.

23. The system of any of examples 18-22, wherein each of the heaters isconfigured to impart a constant incremental phase shift between allpairs of adjacent waveguides of the at least three waveguides.

24. The system of example 23, wherein each of the heaters is configuredto heat a heated region spatially overlapping the at least threewaveguides to a substantially uniform temperature so as to impart auniform phase shift per unit length of heated waveguide, the heatedregion being shaped and positioned such that heated waveguide portionsincrease, between all pairs of adjacent waveguides, by constant lengthincrements.

25. The system of example 24, wherein the at least three waveguides areuniformly spaced and the heated regions are triangular in shape.

26. The system of example 23, wherein spacings between the at leastthree waveguides are selected based on a temperature distributionassociated with the heaters to achieve the constant incremental phaseshift.

27. The system of any of examples 18-27, wherein each of the heaterscomprises a heating filament winding across an area laterallyoverlapping the at least three waveguides in a region to be heated.

28. The system of example 10, wherein the heating filaments comprisesegments of equal and constant width connected by metal connections.

29. The system of any of examples 18-28, comprising a plurality ofbidirectional pairs of heaters, each pair comprising a forward heaterand a backward heater, at least some of the forward heaters or at leastsome of the backward heaters being configured to operate in parallel.

30. The system of any of examples 18-29, wherein the heaters laterallyoverlap with a section of the at least three waveguides in which the atleast three waveguides all have a common curvature.

31. The system of example 30, wherein the heaters laterally overlap withstraight sections of the at least three waveguides.

32. The system of any of examples 18-31, wherein the AWG and the beamsweeper are implemented in a PIC comprising an SOI substrate including asilicon handle, a buried oxide layer disposed on top of the siliconhandle, a silicon device layer disposed on top of the buried oxidelayer, and a cladding layer disposed on top of the silicon device layer.

33. The system of example 32, further comprising a back-etched region inthe silicon handle underneath the bidirectional pair of heaters.

34. The system of example 33, wherein the back-etched region has aconstant width across the at least three waveguides.

35. The system of example 32, further comprising thermal-isolationchannels formed in at least one of the silicon device layer or thecladding layer surrounding heated portions of the at least threewaveguides.

36. The system of example 35, wherein the heaters comprise heatingsegments embedded in the cladding layer.

37. The system of example 36, further comprising temperature-sensingelements embedded in the cladding layer in between the heating segments.

38. The system of example 37, further comprising thermal isolationtrenches in the cladding layer in between adjacent heating segments andtemperature-sensing elements.

39. The system of example 32, wherein the heaters comprise doped,resistive regions in the silicon device layer.

40. The system of any of examples 18-39, wherein the AWG isbidirectional, the system further comprising a second sweeper comprisinga third input FPR, a third output FPR adjoining and optically coupled tothe first output FPR of the AWG, at least three waveguides connectedbetween the third input FPR and the third output FPR, the at least threewaveguides and the third output FPR being configured to focus lightpropagating in the at least three waveguides from the third input FPR tothe third output FPR at a focus at an interface between the third outputFPR and the first output FPR, and at least one bidirectional pair ofheaters laterally overlapping with the at least three waveguides, eachof the heaters configured to impart an incremental phase shift betweenthe light propagating in the at least three waveguides to thereby shifta lateral position of the focus.

41. A system comprising: an AWG comprising a plurality of waveguidesconnected between a first input FPR and a first output FPR; and a beamsweeper for tuning a wavelength response of the AWG, the beam sweepercomprising a second input FPR, a second output FPR adjoining andoptically coupled to the first input FPR of the AWG, at least threewaveguides connected between the second input FPR and the second outputFPR, the at least three waveguides being arranged, in a regionimmediately preceding the second output FPR, along rays emanating from acommon center at the interface of the second output FPR with the firstinput FPR so as to focus light propagating in the at least threewaveguides from the second input FPR to the second output FPR at a focusat the interface, and at least one heater laterally overlapping with theat least three waveguides, each of the heaters configured to impart anincremental phase shift between the light propagating in the at leastthree waveguides to thereby shift a lateral position of the focus.

42. A method comprising: tuning a wavelength response of an arrayedwaveguide grating (AWG) with a beam sweeper comprising an input FPR, anoutput FPR adjoining and optically coupled to an input FPR of the AWG,at least three waveguides connected between the input FPR and the outputFPR of the beam sweeper, and a pair of forward and backward heaterslaterally overlapping with the at least three waveguides, the tuningcomprising: focusing light propagating in the at least three waveguidesfrom the input FPR of the beam sweeper to the output FPR of the beamsweeper at a focus at an interface between the output FPR of the beamsweeper and the input FPR of the AWG, and using one of the forward orbackward heaters to impart an incremental phase shift between the lightpropagating in the at least three waveguides to thereby shift a lateralposition of the focus, wherein the phase shift imparted by the forwardheater is of an opposite sign than the phase shift imparted by thebackward heater, whereby the forward heater and the backward heatershift the lateral position of the focus in mutually opposite directions.

43. A method comprising: tuning a wavelength response of an arrayedwaveguide grating (AWG) with a beam sweeper comprising an input FPR, anoutput FPR adjoining and optically coupled to an input FPR of the AWG,at least three waveguides connected between the input FPR and the outputFPR of the sweeper and arranged, in a region immediately preceding theoutput FPR of the beam sweeper, along rays emanating from a commoncenter at the interface of the output FPR of the beam sweeper with theinput FPR of the AWG, and at least one heater laterally overlapping withthe at least three waveguides, the tuning comprising: focusing lightpropagating in the at least three waveguides from the input FPR of thebeam sweeper to the output FPR of the beam sweeper at a focus at aninterface between the output FPR of the beam sweeper and the input FPRof the AWG, and using the at least one heater, imparting an incrementalphase shift between the light propagating in the at least threewaveguides to thereby shift a lateral position of the focus.

44. A method comprising: in a photonic integrated circuit (PIC)comprising a silicon-on-insulator (SOI) substrate, an arrayed waveguidegrating (AWG) formed at least partially within a silicon device layer ofthe SOI substrate, a pair of forward and backward heaters disposed atleast partially within at least one of a cladding layer or the silicondevice layer and laterally overlapping with the array of waveguides, anda back-etched region formed within the SOI substrate underneath the pairof heaters, altering a wavelength dispersion of the AWG by using one ofthe forward or backward heaters to impart an incremental heat-inducedphase shift between adjacent waveguides of the AWG, the phase shiftimparted by the forward heater being of an opposite sign than the phaseshift imparted by the backward heater, wherein the back-etched regionreduces dissipation of heat generated by the one of the forward orbackward heaters in the SOI substrate.

Although embodiments have been described with reference to specificexample embodiments, it will be evident that various modifications andchanges may be made to these embodiments without departing from thebroader spirit and scope of the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense. The accompanying drawings that form a parthereof show by way of illustration, and not of limitation, specificembodiments in which the subject matter may be practiced. Theembodiments illustrated are described in sufficient detail to enablethose skilled in the art to practice the teachings disclosed herein.Other embodiments may be used and derived therefrom, such thatstructural and logical substitutions and changes may be made withoutdeparting from the scope of this disclosure. This description,therefore, is not to be taken in a limiting sense, and the scope ofvarious embodiments is defined only by the appended claims, along withthe full range of equivalents to which such claims are entitled.

What is claimed is:
 1. A system comprising: a beam sweeper comprising: an input free propagation region (FPR), an output FPR, at least three waveguides connected between the input FPR and the output FPR, the at least three waveguides being configured so as to focus mixed-wavelength light received at the input FPR and propagating in the at least three waveguides from the input FPR to the output FPR at a single focus at an exit surface of the output FPR, and at least one phase tuner configured to impart an incremental phase shift between the light propagating in the at least three waveguides to thereby shift a lateral position of the focus.
 2. The system of claim 1, further comprising: a dispersive grating optically coupled to the beam sweeper at the exit surface of the output FPR.
 3. The system of claim 2, wherein the dispersive grating comprises at least one of an arrayed waveguide grating (AWG), an Echelle grating, or a vertical grating coupler.
 4. The system of claim 1, wherein the system comprises an optical switch comprising the beam sweeper.
 5. The system of claim 1, wherein the at least three waveguides of the beam sweeper are all equal in length and arranged, in a region immediately preceding the output FPR, along rays emanating from a common center point at an exit surface of the output FPR.
 6. The system of claim 1, wherein the at least one phase tuner is configured to impart a constant incremental phase shift between all pairs of adjacent waveguides of the at least three waveguides.
 7. The system of claim 6, wherein the at least one phase tuner comprises a heater configured to heat a heated region spatially overlapping the at least three waveguides to a substantially uniform temperature so as to impart a uniform phase shift per unit length of heated waveguide, the heated region being shaped and positioned such that heated waveguide portions increase, between all pairs of adjacent waveguides, by constant length increments.
 8. The system of claim 1, wherein the at least one phase tuner comprises a bidirectional phase tuner configured to selectively impart the incremental phase shift in one of two mutually opposite directions across the at least three waveguides to thereby selectively shift the lateral position of the focus in one of two mutually opposite directions across the exit surface.
 9. The system of claim 1, wherein the at least one phase tuner laterally overlaps with a section of the at least three waveguides in which the at least three waveguides all have a common curvature.
 10. The system of claim 9, wherein the at least one phase tuner laterally overlaps with straight sections of the at least three waveguides.
 11. The system of claim 1, wherein the beam sweeper is implemented in a photonic integrated circuit comprising a silicon-on-insulator (SOI) substrate including a silicon handle, a buried oxide layer disposed on top of the silicon handle, a silicon device layer disposed on top of the buried oxide layer, and a cladding layer disposed on top of the silicon device layer.
 12. The system of claim 1, wherein the at least one phase tuner comprises multiple phase tuners driven in parallel.
 13. A method comprising: tuning a focus position of a beam sweeper comprising an input FPR, an output FPR, at least three waveguides connected between the input FPR and the output FPR, the at least three waveguides being configured to focus mixed-wavelength light received at the input FPR and propagating in the at least three waveguides from the input FPR to the output FPR at a single focus at an exit surface of the output FPR, the tuning comprising: imparting an incremental phase shift between the light propagating in the at least three waveguides to thereby shift a lateral position of the focus.
 14. The method of claim 13, wherein the output FPR is optically coupled to a dispersive grating, and wherein the focus position of the beam sweeper is tuned to thereby tune a wavelength response of the dispersive grating.
 15. The method of claim 14, wherein the dispersive grating comprises an arrayed waveguide grating (AWG), an Echelle grating, or a vertical grating coupler.
 16. The method of claim 13, wherein the focus position of the beam sweeper is tuned to perform optical switching.
 17. The method of claim 13, wherein the imparted incremental phase shift is constant across all pairs of adjacent waveguides of the at least three waveguides.
 18. The method of claim 13, wherein the incremental phase shift is imparted by heating a region spatially overlapping with the at least three waveguides.
 19. The method of claim 13, wherein the incremental phase shift is imparted selectively in one of two mutually opposite directions across the at least three waveguides to thereby selectively shift the lateral position of the focus in one of two mutually opposite directions across the exit surface.
 20. The method of claim 13, wherein the incremental phase shift is imparted by multiple phase tuners driven in parallel. 